1. Field of the Invention
The present invention relates to a magnetic disk memory system, and more specifically to a magnetic disk memory system in which a magnetic disk of perpendicular recording type is used.
2. Description of the Related Art
Recently, in the field of computers, magnetic disk memory systems are widely used as randomly accessible auxiliary memory systems having large capacities. As the use of the memory system widens, there is an increasing demand for a larger memory capacity and higher recording density of the magnetic disk memory system.
The magnetic disk memory system includes a plurality of magnetic disks each prepared by forming a magnetic recording layer on a non-magnetic substrate, and arranged one on another around a common rotation shaft. The memory system also includes an arm on which a magnetic head for writing or recording data on these magnetic disks, and reading or reproducing data therefrom, is provided. Further, the memory system has an actuator for aligning the magnetic head to a predetermined location of a magnetic disk.
In conventional magnetic disk memory systems, the magnetic head is set at the location so as not to be brought into direct contact with the surface of the disk which is rotated at high speed during writing or reading of data. Specifically, the magnetic head accesses a given position of the surface of the disk while flying slightly above the surface, and writing or reading of data is carried out with respect to tracks arranged on the disk surface in such a manner as a concentric circle.
With regard to such magnetic disk memory systems as described above, many studies have been done in order to meet the demand for an increase in memory capacity. For example, these studies include the improvement of the linear recording density (that is, recording density in track direction) of a disk, and the increase in track density. Further, recently, the contact recording is intensively studied as a way of raising the recording density. In the contact recording, writing or reading of data is carried out in a low flying state in which the flying height of the magnetic head with respect to the surface of the magnetic disk is rendered extremely low, or in a state in which the magnetic head is brought into substantial contact with the magnetic disk.
Moreover, in 1975, the perpendicular magnetic recording system which utilizes magnetic anisotropy in a direction perpendicular to the disk plane was proposed in order to increase the linear recording density. With this perpendicular magnetic recording system, recording at a remarkably higher density can be performed than a longitudinal magnetic recording system in which magnetic anisotropy in its plane direction is utilized. This is because in the perpendicular magnetic recording system, the demagnetizing field at a magnetization reversal region is very small in principle, thus enabling to narrow the magnetization transition width. In the perpendicular magnetic recording system, a perpendicular magnetic recording head having a strip-shaped soft magnetic thin film, with which a recording magnetic field having better orientation in a perpendicular direction can be obtained, is known to be effective in raising the recording density. Further, a magnetic disk having a double-layered perpendicular magnetic recording medium structure in which a soft magnetic backing layer is provided underneath a perpendicular magnetic anisotropy recording layer, has been proposed for the purpose of enhancing the efficiency of writing and reading in the perpendicular magnetic recording system, as well as achieving a more sharp magnetic reversal. With this magnetic disk, demagnetization field at the tip end of the magnetic head can be reduced by the magnetic interaction between the magnetic head and the soft magnetic backing layer, and therefore a large recording magnetic field can be obtained. Similarly, the demagnetization at the tip end of the head can be reduced also during reproduction of the data, and therefore the effective magnetic permeability can be improved. Consequently, the magnetic flux from the magnetic disk can be efficiently focused on the magnetic head, thereby obtaining a large reproduction signal.
More recently, in the perpendicular magnetic recording system, an active-type magnetic head employing a magneto-resistance element (MR element) serving to increase the sensitivity of the signal reproduction has been proposed. The active-type magnetic head serves to convert a magnetic flux supplied from a recording medium into an electric signal by taking advantage of the property inherent to the MR element, in which the electrical resistance of the MR element made of a soft magnetic material such as permalloy changes due to an external magnetic field. The reproducing sensitivity of a magnetic head using an MR element is proportional to the amount of current (sense current) supplied to the MR element for converting the change in resistance of the MR element into the change in voltage. Therefore, if the relative speed between the magnetic head and a medium is low, a large output can be obtained by increasing the sense current. By using this large output, the track width can be narrowed, and the track density can be increased.
Examples of the magnetic head comprising a built-in MR element, which is designed for use in the perpendicular magnetic recording system, are disclosed in, for example, Jap. Pat. Appln. KOKOKU Publications Nos. 62-24848 and 63-67250. In the magnetic heads disclosed in these documents, the MR element is located adjacent to the single-pole film used for recording, and disposed to face a recording medium.
However, in the case of these magnetic heads, the MR element contact with the medium causes it to be abraded. Consequently, the cross section of the MR element decreases, and accordingly the resistance of the element increases. As a result, the produced output is varied.
Jap. Pat. Appln. KOKOKU Publication No. 53-25488 provides a structure for avoiding the abrasion of an MR element, and discloses a magnetic head comprising two soft magnetic yokes arranged in contact with a medium and the MR element disposed in a bridge manner in parallel with the yokes. In such a magnetic head, the MR element is not abraded since it is never brought into contact with the surface of a medium. However, the direction of the magnetic field generated during recording is in parallel with that of the film surface of the MR element. Therefore, a large recording magnetic field is applied in the direction of the film surface of the MR element during recording, thereby disturbing the magnetic domain structure of the MR element. In addition, this magnetic head is not applicable to the perpendicular magnetic recording system because this magnetic head serves as a ring-type head during recording, thus generating a large magnetic field in the longitudinal direction.
The influence of the recording magnetic field on an MR element can be removed by arranging the recording head and the reproducing MR element apart from each other. However, in this case, a distance between the recording head and reproducing head is likely to create a track error, thus remarkably deteriorating the quality of signals. Such a problem is more likely to occur particularly in a track of a small diameter.
On the other hand, in a magnetic head in which the MR element is placed adjacent to the recording single-pole film and the magnetic shield film, the MR element and these soft magnetic films (single-pole film and magnetic shield film) are magnetically coupled during reproduction of signals. When the distance between the MR element and these soft magnetic films is designated by g and the film thickness of the MR element is designated by Tm, the resolution of the magnetic head is limited by the sum of these values (g+Tm). As a result, despite that a magnetic recording layer of a perpendicular magnetic anisotropy is used, a high reproducing resolution equivalent to a good recording resolution of that magnetic recording layer is not likely to be obtained.
Moreover, in order to obtain a large reproduction signal, a thin MR element having a thickness of 0.1 .mu.m or less, preferably 0.05 .mu.m or less, should be used since the resistance value of the MR element must be increased. In this case, the film thickness of the MR element will be substantially the same as the grain diameter of crystals constituting the magnetic recording layer of the perpendicular magnetic anisotropy. As a result, not only the resolution cannot be increased to a sufficient level, but also the noise due to the grain diameter of the crystals is increased, thereby lowering the signal quality.
In the above-described magnetic disk memory system, as shown in FIG. 1, a head slider formed by mounting a magnetic head 539 in a part of a slider 538 is used. The head slider is arranged so that air-bearing surfaces 540 consisting, of two parallel planes face the surface of a magnetic disk, and a gap portion 541 of the magnetic head 539 is so located as to face the magnetic disk surface for recording and reproduction of a signal.
FIG. 2 is a conceptual diagram illustrating the operation principle of the head slider. A slider 542 is supported and restrained on a supporting point 543 under the condition that a force (indicated by arrow 544) perpendicular to the surface of a disk is imposed and a moment (indicated by arrow 545) around the supporting point is imposed by means of a gimbal spring and a suspension spring. A magnetic head 546 is provided on the rear end of the slider. A gap 547 is located at the rear end of an air bearing surface 548 of the slider such as to face the surface of a magnetic disk 550 which rotates in the direction indicated by arrow 549. This head slider mechanism is also employed in the present invention, and details thereof will be described later. As regards the structure in which such a head slider is connected to an actuator, see FIG. 20 and the description concerning the figure, later provided. The embodiment shown in FIG. 20 is similar to the conventional art.
Referring to FIG. 2, when the magnetic disk 550 rotates, the surrounding air moves along with the disk due to the viscosity of air itself, generating an air flow. This air flow creates a hydrodynamic pressure between the air bearing surface 548 and the surface of the magnetic disk 550. The head slider 542 flies above the disk with a constant clearance 551 therebetween where the hydrodynamic pressure and the supporting constraint force of the gimbal and suspension springs are balanced with each other.
In this mechanism, the magnetic head 546 is not brought into contact with the magnetic disk 550, and therefore the abrasion of the head and disk, caused by the contact therebetween, can be prevented. However, from a view point of magnetic recording, because of the clearance gap 551 between the magnetic head and the magnetic disk, a read and write spacing loss is created, thus lowering the output of signals. This problem is particularly prominent in the case where the recording density is high and the wavelength of a recording signal is short, or in the perpendicular magnetic recording system in which the easy axis of magnetization is perpendicular to the disk surface. Therefore, this problem blocks the increasing in recording density.
In order to overcome this problem, there has been proposed a recording/reproducing system in which the clearance gap between the magnetic head and the magnetic disk is made so small that they are substantially in contact with each other. For example, Jap. Pat. Appln. KOKAI Publication No. 3-178017 discloses a method in which a magnetic head is shaped into a stylus, and the recording/reproducing portion of the tip end of the magnetic head is pressed on the disk with an extremely light load. This publication states that the amount of abrasion of a head, caused by sliding of the head on a disk, can be suppressed in a practically acceptable range by means of imposing extremely light load on a extremely minute area. However, it is not easy to stably apply a constant and extremely light load on the stylus head.
As a method of stably reducing the distance between a magnetic head and a magnetic disk, there is proposed, for example, in Jap. Pat. Appln. KOKAI Publication NO. 62-3476, a method in which a large and a small flying sliders are coupled, and a magnetic head is mounted in the small slider. However, with this method, the small slider also flies, and therefore the gap between the magnetic head and the magnetic disk can only be reduced to a limited level. Consequently, it is difficult to bring the magnetic head and the magnetic disk into contact with each other.